As a method of forming images using thermal transfer, a thermal diffusion dye transfer method (sublimation dye transfer printing method) of superimposing a thermal transfer sheet in which a thermal diffusion dye (sublimation dye) as a recording material is supported on a base of a plastic film or the like on a thermal transfer image-receiving sheet in which a layer receiving the dye is provided on another base sheet of paper, a plastic film or the like to form a full color image. Since this method uses a thermal diffusion dye as a coloring material, a density and a tone can be freely adjusted on a dot by dot basis and a full color image which is faithful to a source document can be sharply displayed on the image-receiving sheet, and therefore this method is applied to the formation of color images of a digital camera, a video recorder, a computer, and the like. These images are of high quality equivalent to a silver halide photo.
However, in the formation of images by the sublimation dye transfer printing method, a problem that conventional thermal transfer sheets cannot attain sufficient print densities has arisen as a printing speed of a thermal transfer printer are becoming increasingly high.
It was also thought to increase a ratio of a dye to a resin (Dye/Binder) in a dye layer of the thermal transfer sheet for the purpose of improving print density and transfer sensitivity in printing, but if this ratio is increased, the dye is shifted to a heat resistant slipping layer on the backside of the thermal transfer sheet during winding and storing the thermal transfer sheet, and this shifted dye is kicked back to a dye layer of different colors to contaminate the layer when the thermal transfer sheet is wound off. When this contaminated layer is thermally transferred to the image-receiving sheet, there were problems that the image-receiving sheet has the hue different from a designated color or the so-called scumming is produced. Further, when high energy is applied in a thermal transfer printer at the time of thermal transfer in forming images for the purpose of improving print density and transfer sensitivity in printing, the thermal adhesion of the dye layer to the receiving layer occurs and the so-called abnormal transfer tends to occur. When a large amount of a release agent is added to the receiving layer in order to prevent the abnormal transfer, there was a problem of producing the blur/scumming of images.
In the formation of images by the sublimation dye transfer printing method, it is required that the adhesion strength of the base sheet to the dye layer in the thermal transfer sheet is high in order to prevent the so-called abnormal transfer, in which the whole dye layer is transferred to the thermal transfer image-receiving sheet.
As a thermal transfer sheet in which the print density is high and the adhesion strength of the base sheet to the dye layer is improved, a thermal transfer sheet in which an intermediate layer is provided between the base sheet and the dye layer is known.
As the thermal transfer sheet provided with the intermediate layer, there are known, for example, a thermal transfer sheet in which a hydrophilic barrier consisting of polyvinylpyrrolidone and poly vinyl alcohol/under coat layer is provided between a dye layer and a base sheet, and a thermal transfer sheet in which an intermediate layer containing a sublimation dye having a diffusion coefficient smaller than that of a sublimation dye contained in a recording layer is provided between a base film and the recording layer containing a sublimation dye (See, for example, Japanese Kokai Publication Hei5-131760 and Japanese Kokai Publication Sho60-232996). However, a printed substance having an adequately high print density cannot be obtained in any thermal transfer sheet.
In Japanese Kokai Publication Sho59-78897, a thermal transfer sheet, in which a layer formed by vapor deposition of metal or metal oxide is formed on a base and on this layer, a thin layer of dye is provided, is described. However, there was a problem that this thermal transfer sheet cannot attain a printed substance having an adequately high print density, and it requires special equipment in vapor deposition and a production cost becomes high.
In Japanese Kokai Publication 2003-312151, a thermal transfer sheet, in which a good adhesive layer containing a homopolymer of N-vinylpyrrolidone or a copolymer of N-vinylpyrrolidone and another component is provided between the base and the dye layer, is described. This good adhesive layer may be a substance formed by mixing alumina, silica and like in addition to the polymers described above, but it is not essential to contain these compounds. In the thermal transfer sheet of Japanese Kokai Publication 2003-312151, there is a problem that the efficiency of dye transfer is insufficient and in addition a releasing property in printing is low and further deteriorated when being stored in the conditions of high temperatures and high humidity.
In Japanese Kokai Publication Sho63-135288, an example, in which trialkoxysilane is applied as an under coat layer to an interface between the base and the dye layer of a thermal transfer sheet, is described, but a problem that a dye-donating element adheres to a receiving element after printing a dye on a thermal transfer image-receiving sheet and therefore the releasing property is low is pointed out. This under coat layer is unstable against water and is prone to hydrolysis since the above-mentioned silane compound has an alkoxide group, and has a problem of deteriorating a dye in the dye layer. Furthermore, in Japanese Kokai Publication Sho63-135288, there is no description on mixing with other oxides.
In Japanese Kokai Publication Hei5-155150, an under coat layer formed by reacting a polymer having an inorganic primary chain comprising oxide of Group IVb metal with a copolymer such as acryloxyalkoxysilane is described. The under coat layer in Japanese Kokai Publication Hei5-155150 has a problem that it is low in heat resistance since it is an organic chain derived from the above copolymer and that it is prone to hydrolysis and unstable since it has the above inorganic primary chain. In addition, with respect to silicate, in Japanese Kokai Publication Hei5-155150, silicon is only presented as of a Group IVb metal, and there is no further specific description and no description on mixing of silicate with other oxides.
Japanese Kokai Publication Hei5-131760
Japanese Kokai Publication Sho60-232996
Japanese Kokai Publication Sho59-78897
Japanese Kokai Publication 2003-312151
Japanese Kokai Publication Sho63-135288
Japanese Kokai Publication Hei5-155150